1. Field of the Invention
The present invention relates to an automotive automatic air conditioning system with a variable displacement compressor, particularly to a system which provides high dehumidification performance under conditions of high humidity and relatively low ambient temperature, such as a rainy spell in autumn.
2. Description of the Prior Disclosure
Recently, there have been proposed and developed various automatic air conditioning systems with a control unit which controls the opening angle of doors, such as a fresh/recirculation switchable air intake door, an air mixing door, a defroster door, a chest vent door, a foot vent door, or the like, and controls the amount of air flowing through the evaporator of the air conditioning system in response to output signals from various sensors for detecting various physical quantities, such as an ambient temperature, room temperature in the vehicular cabin, magnitude of insolation, intake air temperature of the evaporator, suction pressure of the compressor, and in response to output signals indicative of an ON/OFF state of various switches, such as an air conditioner switch, a blower switch, an ignition switch, a defroster switch, and so forth. In general, such automatic air conditioning systems control the discharge of refrigerant from the compressor depending upon the intake air temperature of air flowing through an evaporator, thereby preventing the evaporator from freezing. Conventionally, the intake air temperature is measured just behind or in the vicinity of the evaporator. One such air conditioning system has been disclosed in Japanese Utility Model First Publication (Jikkai) Showa 59-79410.
As is well known, the suction pressure of a compressor is correlatively lowered in accordance with the lowering of the intake air temperature of the evaporator under conditions of relatively high ambient temperature. Therefore, an automatic air conditioning system conventionally controls the above mentioned suction pressure of the compressor in such a manner that the suction pressure is kept higher than a preset pressure which is determined on the basis of a threshold value at which the evaporator starts to freeze. The threshold value of the intake air temperature will be hereinafter referred to as a freezing start possible temperature . In prior art air conditioning systems, to avoid freezing of the evaporator, that is, to prevent the suction pressure of the compressor from becoming lower than a preset pressure, the air conditioning system controls the suction pressure in such a manner that, if the suction pressure of the compressor becomes lower than the preset pressure, the discharge from the compressor is lowered and thus the cooling power of the system is lowered.
However, in such conventional air conditioning systems for automotive vehicles, since the correlation between the intake air temperature of the evaporator, the refrigerant temperature measured at the suction side of the compressor and the suction pressure of the compressor often cannot be maintained due to ice or frost adhering to the evaporator under low ambient air temperatures, the compressor cannot be sufficiently controlled by the intake air temperature. For this reason, when the vehicle is quickly accelerated under the above mentioned conditions of low ambient air temperature, the suction pressure and the refrigerant temperature at the intake side of the compressor are rapidly lowered, thereby causing freezing of the evaporator or damage to the compressor.
If the compressor is stopped when the ambient air temperature reaches a temperature at which frost will begin to adhere to the evaporator, the dehumidification function of the evaporator is not performed at all. Therefore, occupants of the vehicle will feel uncomfortable under conditions of high humidity and low ambient temperature.